1. Field of the Invention
The invention relates to a buttonhole sewing machine, comprising a needle which is drivable in up and down reciprocation; at least one work piece clamp which is displaceable in a y direction; a buttonhole cutting device, which is disposed downstream of the needle in the y direction, which comprises a knife, which comprises a cutting block that cooperates with the knife, and a cutting drive for motion of the knife and the cutting block relative to each other by variable cutting force.
2. Background Art
U.S. Pat. No. 4,552,080 A describes a buttonhole sewing machine of the generic type. In this case, operation of the cutting block takes place by a pneumatically actuated piston-cylinder drive which can be actuated by varying pressure for the generation of varying cutting forces. It is also possible to vary the speed of cutting. Drawbacks of this familiar design reside in that precisely defined cutting forces are very difficult to produce.
It is an object of the invention to develop the known buttonhole sewing machine in such a way that the cutting force can easily be adjusted in a precisely reproducible manner.
According to the invention, this object is attained by the features wherein the cutting drive includes several linear drives, which are connected in parallel; and which are pneumatically actuated selectively. By providing several pneumatically actuated linear drives which are connected in parallel and can selectively be operated by the same pressure, the cutting force can be set and selected in several steps, each of which being precisely reproducible. This ensures highly accurate adaptation to varying cutting conditions which are influenced by the hardness and type of work piece, the number of work piece layers to be cut, but also by the shape and/or size of the incision that is to be made.
In keeping with an embodiment wherein the linear drives are multichamber cylinders, with a piston being disposed in each chamber, the pistons being mounted on a joint piston rod; wherein at least one chamber is provided with a piston that is bilaterally actuated; and wherein three chambers are provided with pistons which are jointly actuated unilaterally in the same direction of motion, the linear drives are used as active drives for the generation of the cutting force.
In the advantageous embodiment according to which the cutting drive includes a piston-cylinder drive which supports itself against the linear drives that are designed in the form of force limiters; and according to which the force limiter comprises several diaphragm cylinders as linear drives, the linear drives are employed passively i.e., depending on the actuation of the linear drives, the force limiter they cooperate to form constitutes a more or less resilient abutment for the piston-cylinder drive that generates the cutting force. The piston-cylinder drive always produces at least the maximally possible cutting force, part of which may then be compensated by the force limiter.
Operation of the linear drives takes place via multiple-way valves which are triggered by a central operating unit.
Details of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing.